The VENUS observatory measures the tide and large surface waves with our pressure sensors, but internal waves also exist and can sometimes been seen in the inverted echo-sounder data. Here is a one hour record of the 200 kHz ZAP inverted echo-sounder data from the Delta Dynamics Laboratory (DDL) on 11 January 2012 (20:00 UTC). The train of solitary internal waves shows up as an undulating layer of scattering material in the upper 20m.

Internal waves are generated when an “internal” density interface is disturbed. The ocean is stratified in density, with dense sea-water always underlying less dense (lighter) sea-water. Sea-water density varies as a function of both temperature and salinity, with colder water typically being more dense than warmer water, and more saline water having a higher density than fresher water. Throughout most of the Salish Sea, salinity dominates as the dictating parameter for determining sea-water density. The influence of the Fraser River and other coastal rivers is to produce a relatively thin (10-30m) layer of relatively fresher sea-water near the surface. The density difference at the boundary between the fresher surface layer and the deeper salty layers is ideal for supporting internal waves. In this image we can also see that the troughs are associated with surface convergence, and tend to accumulate near-surface flotsam and bubbles (elevated scattering material/back-scatter). From above, we can often see the surface signature of these internal wave troughs as long bands of flotsam and concentrated capillary waves (roughness), separated by smooth surface water over the divergent crests of the internal waves.